A communication scheme in which millimeter waves in a 60 GHz band are used as a carrier for transmitting more information is attracting attention. Information is transmitted in units of frames. When the temporal lengths of frames are the same, more information is transmitted by using a high-frequency carrier than a low-frequency carrier.
In the case of transmitting information by using a high-frequency carrier, the amount of information transmitted per unit time is larger, and the influence of noise in a transmission path is larger than in the case of transmitting information by using a low-frequency carrier. If noise increases and degrades the state of a communication link, thereby causing difficulty in communication, the amount of information that is lost per unit time is very large.
Thus, it is desirable that the state of the communication link be detected on a receiver side, and the result of the detection be quickly fed back to a transmitter, thereby performing information transmission adapted to the state of the communication link. In the related art, however, information is transmitted/received by using only one certain carrier. Accordingly, the effect of the noise environment or the like may degrade the states of both links for transmission and reception. If a transmission-disabled state occurs in the communication link, then neither transmission nor reception of information can be performed.
Furthermore, in a frame constituted by a transmission bit string, it is not assumed that transmission thereof is stopped before completion. Even if the state of a link degrades during transmission of a frame, feedback for the state of the link can be performed only after transmission of the frame has been completed. Therefore, quick feedback cannot be performed.
Patent Literature 1 describes a typical method for transmitting/receiving one or plural packets, calculating an error rate for each packet, and requesting retransmission. In this method, however, communication quality cannot be improved during transmission of a long packet (jumbo frame) by dynamically changing a parameter, such as the power of a transmitter or an attenuation amount.
Patent Literature 2 relates to a system for transmitting two different pieces of information, initial information and additional information. In this system, the initial information is transmitted through a high-speed line, and the additional information is transmitted through a low-speed line. Here, the additional information may be a part of initial information that has not been transmitted correctly and needs to be retransmitted.
The system according to Patent Literature 2 is directed to decreasing the occupancy of a high-speed line by using a low-speed line to retransmit a non-urgent application. However, this system does not employ an approach of dynamically changing a parameter, such as the power of a transmitter or an attenuation amount, during transmission of a long packet (jumbo frame) to improve the state of the link, thereby improving communication quality.
Patent Literature 3 discloses a system for measuring an influence of a multipath and thereby changing a symbol rate or the level of multi-level modulation in a transmitter and a receiver using multiple-input multiple-output (MIMO). This system is configured under the assumption that the transmitter and the receiver face each other and are capable of performing equivalent communications.
Regarding determination of a symbol rate or the level of multi-level modulation, Patent Literature 3 describes a method in which a high symbol rate is used in an initial stage of communication and the symbol rate is decreased if the influence of the multipath is large. In addition, there is the following description: “The above process can be carried out prior to the start of communications. However, even during data communications, the bit error rate, the frame error rate, the packet error rate, and the retransmission request rate (retransmission rate) may be monitored, and the symbol rate may be lowered to make the numerical values of these rates sufficiently lower.”
However, if the symbol rate of the transmitter changes, it is difficult for the receiver to quickly recognize the change in symbol rate and quickly adapt so as to normally perform reception at the new symbol rate. Patent Literature 3 does not describe improving communication quality during transmission of a long packet (jumbo frame) by dynamically changing a parameter, such as the power of the transmitter or an attenuation amount.